1. Field of the Invention
The present invention relates to the detection of a water film formed on certain parts of a printing machine and is applicable likewise to detect a water film or an oil film on steel products or in steel making machines or the like.
2. Description of the Related Art
In a printing machine (comprising a group of ink and water applicator rollers, plates, and various cylinders), the quality of printing is affected by the physical conditions of a water film formed on a plate. FIG. 6 is a schematic diagram of a water film sensor according to a conventional design which detects a water film on a plate 6. With such a sensor, water film detection is carried out as described in the following.
Infrared rays from an infrared light source 15 are gathered by an infrared collector lens 16 and passed through interference filters 22 and 23 disposed on a chopper, whereby a reference light beam and a light beam with an absorption wavelength of water may be selected alternately. Then, the light beams are guided by a photo waveguide 18 and absorbed by the water film on the plate 6, and then the light beams which have passed through the water film and have been reflected from the plate 6 pass through a receiver lens 19 and is converted into signals by a photo sensor element 20. Using a position-detecting means 21 for the interference filters 22, 23, the signals from the photo sensor element 20 are identified. A signal processor 9 is fed with these signals and positional information from the position-detecting means so that the processor may process data and convert them to a water film thickness.
The following are examples of printing quality problems associated with on-plate water films:
(1) Water lining
Water lining is a phenomenon in which a plurality of streaks (varying ink density) with a width of 1.5 mm or less occur.
(2) Web staining
Web staining occurs over a wide area where printing pictures or patterns do not exist (a portion without picture or line) on a plate.
The measurement of water film thickness for the case in paragraph (2) above covers a wide area and can be done with the measurement of the mean water film thickness over the range of stain. Therefore, even if the chopper speed is low, the conventional method of water film thickness measurement is serviceable in this case. However, the area of water film measurement in the case of paragraph (1) above is small, and a high chopper speed is required.
Thus, the prior art described above is confronted with the following problems:
(i) Since the infrared light source is not a point source, it is not possible to reduce the cross-sectional diameter of the light beam to less than 1.5 mm without reducing the intensity of light.
(ii) If the area of measurement for water film thickness is smaller, it then becomes necessary to sample with a high-speed chopper. However, conventional mechanical choppers do not have such a high speed.